Heretofore, a polyether polyol as a material for polyurethane elastomers, adhesives, sealants, etc., has been produced by polymerizing an alkylene oxide such as ethylene oxide or propylene oxide by means of an initiator having active hydrogen atoms. As a typical polymerization catalyst for an alkylene oxide, a composite metal cyanide complex (hereinafter referred to also as a DMC catalyst) is known. The DMC catalyst is a catalyst comprising an organic ligand and a metal salt and is represented by a compound having an organic ligand, water and ZnCl2 coordinated on zinc hexacyano cobaltate (Zn3[Co(CN)6]2).
In recent years, it has been reported that the catalytic life of a DMC catalyst is remarkably prolonged by the use of tert-butyl alcohol as an organic ligand (JP-A-4-145143). Further, JP-A-2000-513389 discloses that by employing a highly active DMC catalyst using an organic ligand such as tert-butyl alcohol, it is possible to reduce the amount of the catalyst to be used, and it is possible to eliminate a step for removal of the DMC catalyst after the production of a polyether polyol.
And, JP-A-9-132630 discloses that it has been found that a polyether polyol having from 10 to 1,000 ppm of a residue of a DMC catalyst, or a composition comprising such a polyether polyol and an antioxidant, has a storage stability, and with an isocyanate group-terminated prepolymer prepared by using such a polyether polyol or composition, the storage stability is not poor. Further, in JP-A-9-132630, a triester of phosphorous acid is exemplified as an antioxidant. However, in order to obtain a sufficient storage stability, it is required to use the antioxidant in a large amount as much as at least 500 ppm.
On the other hand, it is known, for example, by JP-A-6-502674 (U.S. Pat. No. 3,391,101) that a phosphorus compound is used in a purification step after preparing a polyether polyol by means of a DMC catalyst having a relatively low activity, using glyme or the like as an organic ligand. JP-A-6-502674 discloses a process wherein a polyether polyol is produced using about 500 ppm, based on the finally formed polyether polyol, of a DMC catalyst, and then the DMC catalyst is decomposed by an alkali catalyst such as an alkali metal hydroxide, followed by neutralization treatment by means of a phosphoric acid compound, whereupon the neutralized salt is removed. Thus, it has already been known to use a phosphoric acid compound to neutralize an alkali used to deactivate the DMC catalyst in the process for removing the DMC catalyst.
Further, JP-A-5-170857 proposes that a thermally stable prepolymer can be produced in the presence of from 100 ppm to 10% of an ester of phosphorus acid during the preparation of the prepolymer by using the obtained polyether polyol.